Mega-Doughnuts: CERN to Study Plan for 100-Kilometer Atom-Smashers

If you thought atom-smashers couldn't get any bigger, think again. European particle physicists will draw up plans for a pair of circular particle colliders, to be built one after the other, that would measure 80 to 100 kilometers in circumference and smash particles at unprecedented energies, officials at the European particle physics laboratory CERN announced today. The goal is to come up with the first "conceptual designs" in 5 years, in time to inform the next revision of the European particle physics strategy, which was formulated in 2006 and revised last May.

The plan would offer an alternative to the current widely held vision for the global future of particle physics, in which the next great collider would be an arrow-straight linear collider, not a circular one. "We have to make a choice at some point," says Patrick Janot, a physicist at CERN, located near Geneva, Switzerland. "Either we have to go linear or we have to go circular."

CERN already has the highest energy atom-smasher: the 27-kilometer-long Large Hadron Collider (LHC), which fires protons into protons. In 2012, it blasted into existence the long-sought Higgs boson, the key to physicists' explanation of how other fundamental particles get their mass. Physicists hope that the LHC, which is now undergoing repairs, will discover other new things when it powers up again in 2015.

The LHC is expected to do its work by 2030, however, and for decades physicists have generally planned that the next accelerator would be a roughly 30-kilometer straight-shot linear collider. It would fire electrons into positrons to create cleaner collisions and study in detail whatever was discovered by the LHC. (There are two distinct designs for the proposed linear collider: the International Linear Collider (ILC) and the Compact Linear Collider (CLIC).)

But in the past few years, interest has been growing in an alternative path. In it, CERN would build a circular electron-positron collider called TLEP roughly three times as long as the LHC. After running that machine for a couple of decades, CERN researchers would then reuse the tunnel for a new proton collider that could reach energies seven times higher than the LHC, opening a new realm of high-energy exploration. The new Future Circular Colliders program aims only to develop those plans to a level comparable to plans for a linear collider, says James Gillies, CERN's spokesman. "It's not about digging holes in the ground now or asking governments for money," Gillies says, "it's just about considering the technology that would be available in 20 or 30 years."

If CERN officials eventually opted for the new plan, they would be repeating a recycling strategy that paid out handsomely for them in recent decades. In the 1980s, CERN built a lower energy electron-positron collider called LEP, which cost 1.3 billion Swiss francs and cemented much of physicists' prevailing standard model by studying particles called the W and Z bosons in detail. After LEP shut down in 2000, physicists then reused the LEP tunnel to house the LHC, which cost 6 billion Swiss francs and turned on in 2008, saving a billion or two in construction expenses. By putting two machines consecutively in the same tunnel, the new plan would seek to realize similar savings, CERN’s Janot says. The tunnel was "by far" the most expensive part of building LEP, he says, "and so it would be for TLEP."

The real question would be whether TLEP would be as fruitful as a linear collider. Because electrons racing in a circle radiate x-rays like mad, a circular machine could not reach as high an energy as a linear collider could—which was the whole reason for planning such a machine. So if the LHC coughs up new particles that are heavier than about 400 times the mass of a proton, then only a linear collider could make them, says Alain Blondel, a physicist at the University of Geneva.

But if all the LHC finds is the Higgs, then the search for new physics will shift to studying it and other familiar particles in great detail. Physicists would hunt for unusual decays and small deviations from the particles' predicted properties, which could be signs of particles just beyond the LHC's reach. In that case, the circular machine would be ideal, as it could produce much more data, including 10 to 30 times as many Higgs as a linear collider, Blondel says.

Of course, all of these considerations come into play only if the circular machines scenario is really technologically feasible. And even if it is, changing course to a circular machine may be a wrenching process, Blondel says. "Many researchers have put a lot of effort into the linear colliders," he says. Still, with its two-for-one economy, the new plan may be very attractive. After all, a billion saved is a billion earned.